The present invention relates generally to a method for machining a work piece into a predetermined shape. More specifically, the invention relates to a method for high speed CNC (Computer Numeric Control) machining which, while suitable for other purposes, is especially useful for rough milling machine operations where a high rate of material removal is desired.
A typical CNC milling machine includes a table adapted to hold the work piece, a chuck for holding a cutter, and means for selectively moving the cutter in three dimensions or axes to engage and cut the work piece. An electronic controller controls the orientation, path, and speed of the cutter according to a computer control program to machine the desired shape or cut-out in the work piece.
Commercially available CAM (Computer Aided Manufacturing) programs aid a programmer in generating control programs for the CNC machine. The programmer typically either generates a surface model of the desired part or enters dimensions for the finished part into the CAM program. The cutter path is then determined by considering additional data such as cutter sizes, operational characteristics of the machine, and machining characteristics of the work piece material. With the aid of the CAM program, the programmer generates a control program for the CNC machine, defining a cutter path or series of paths for machining the work piece, and the associated cutter feed rate and cutter engagement with the work piece.
Prior methods for milling, for example, a cavity in a work piece, typically consist of programming the machine to take a series of parallel cuts in the work piece, each cut being a predefined depth from the surface of the work piece. The depth of each cut and the speed of the cutter are determined by known methods to maintain the stress in the cutter within an acceptable limit. The cutter then follows a predetermined path to remove the material from the work piece to define the desired cavity to that depth. Typically, the control program is written so that the cutter follows the general shape of the cavity, traveling along a path that moves either outwardly (if starting from the center of the cavity) or inwardly (if starting from the edge of the cavity) upon each pass around the cavity. Alternately, the cutter may travel back and forth across the part, from one edge of the cavity to the other, until the material is removed from that depth of the cavity. The depth of the cutter is then increased in a step-by-step manner, the material being removed at each depth before proceeding to the next cut. In some instances, machining stock may be left on the sides of the machined cavity for a final finish-machining operation.
The cutter drive and control mechanism of a CNC milling machine have an inertia associated with changes in speed, direction, and feed rate. Thus, when the cutter is traveling at a relatively high feed rate, as is typical during high speed machining, the control program generated according to prior machining methods typically reduces the speed of the cutter upon approaching a turn in the cutter path to permit the machine to execute the desired change in direction of the cutter within the acceleration limits of the machine. As a result, the feed rate of the cutter and therefore the rate of material removal is temporarily reduced each time the machine executes a turn according to prior methods. And where the cutter path includes multiple turns, such as when machining a cavity in a work piece, the reduction in overall removal rate may be substantial.
Additionally, it is well known that there is typically an increase in engagement between the cutter and the work piece as the cutter approaches and enters an inside corner or relatively sharp turn in the work piece. Such increase in engagement may result in a relatively large increase in stress or load on the cutter, thereby reducing the machining life of the cutter. Reducing the feed rate of the cutter aids in controlling the stress in the cutter resulting from this temporary increased cutter engagement. Again, however, the rate of material removal decreases as the feed rate of the cutter decreases.
In some instances, the programmer may generate a control program which attempts to generally maintain a constant feed rate as the cutter changes directions. However, even with an especially quick or responsive machine, if the cutter is moving at a relatively high speed, the cutter will typically deviate from the programmed path upon execution of the quick change in direction. Such deviation from the programmed path results in excess stock remaining in unknown or not well defined areas, and in the removal of additional material in other undefined areas. As a result, the engagement and load in the cutter during the next pass of the cutter may increase to an unknown level. These considerations may cause the programmer to provide for a programmed feed rate which is less than optimum to account for the possible increase in load on the cutter.
There is an ever present need to increase productivity of facilities and machinery. Thus, there is a need for an improved method of machining which overcomes the above-identified difficulties and uncertainties in the prior methods of programming for a high speed CNC machining to provide for increased metal removal rates. Specifically, there is a need to provide for high speed machining without overloading the tool and without the need to slow the feed rate upon changing direction of the cutter, thus providing for at a constant cutter feed rate and optimum rate of material removal.
The general aim of the present invention is to provide a new and improved method of machining which achieves a higher rate of metal removal as compared to prior machining methods of the same general type.
A detailed objective is to achieve the foregoing by providing for a cutter path which will permit the machine to operate at a substantially constant cutter feed rate throughout the machining operation including changes in direction of the cutter path.
Another detailed objective of the invention is to provide for a cutter path which will permit substantially constant angular engagement between the cutter and the work piece, such engagement not exceeding a predetermined value to avoid premature wear of the cutter.
The invention also resides in the novel method for cutting nested pockets in the work piece where a cavity is desired, each pocket generally expanding the size of the opening in three dimensions by a predetermined distance from the desired cavity.
These and other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.